(1) Field of the Invention
This invention relates to fixing devices, and more particularly it is concerned with a pressure fixing device comprising a pair of pressure rollers allowing a support member supporting a toner image to pass therebetween to effect fixing of the toner image to the support member.
(2) Description of the Prior Art
In image forming apparatus comprising electrophotographic copying apparatus, facsimile systems, printers and the like, a pressure fixing device comprising at least one pair of pressure rolls is commonly used as one of the devices for obtaining fixing of a powder toner image (hereinafter toner image) formed on a support member, such as a sheet of paper, by developing or transfer-printing.
The pressure fixing devices are broadly classified into a 2 roll type shown in FIG. 1 of U.S. Pat. No. 4,192,229, for example, and a 3 roll type described in U.S. Pat. Nos. 4,259,920 and 4,343,234, for example. In ordinary pressure fixing devices, the pressure rolls are subjected to the following treatment to avoid an offset phenomenon. The pressure rolls are provided by subjecting rolls formed of steel to heat treatment to obtain hardening of the material, applying hard chromium plating to the surfaces of the rolls and subjecting the rolls to grinding to provide their surfaces with a coarseness of below 0.8 S (.apprxeq.0.2 .mu.mRa). This gives a smooth and lustrous surface to the image fixed by the pressure rolls obtained by the aforesaid process. However, because of the glare caused by the lustrous surface, the image may be hard to see depending on the angle at which the viewer looks at the sheet carrying the image fixed by the pressure rolls.
Proposals have been made, in order to obviate the aforesaid disadvantage of the prior art and provide an image fixed by pressure rolls which is free from a shiny surface, to treat the surface of the metal roll that comes into contact with a toner image by means of a sand blast to form irregularities on the surface and then apply hard chrominum plating to the surface to provide a coarse surface with rounded, minuscule elevations and depressions (1.3 S-5.0 S), as described in U.S. Pat. No. 4,200,389. U.S. Pat. No. 4,258,095 described a pressure fixing device comprising pressure rolls that have a surface coming into contact with a toner image treated as with silicon carbide to give a coarseness of 0.2-1.2 .mu.m(Ra) thereto.
The pressure rolls of the prior art having coarse surfaces as described hereinabove still have the problem that, because their surface coarseness is 1.2 .mu.m(Ra) at best in view of obtaining excellent results in fixing images while avoiding the occurrence of an offset phenomenon, the luster of a fixed image is not sufficiently reduced to enable the viewer to clearly recognize the image as compared with an image that is thermally fixed. Moreover the pressure fixing devices of the prior art have had the problem that no satisfactory results are achieved in avoiding the occurrence of an offset phenomenon.